JEE Main · 2005 · 08 May 2005 · Offline · Haloalkanes and Haloarenes

The $S_{N}2$ pathway demands that the nucleophile attack the carbon from the side directly opposite the leaving group, straight through the back of the molecule. In a tertiary alkyl halide that carbon is ringed by three bulky alkyl groups, and they physically block this backside approach. The resulting steric hindrance is what makes tertiary halides practically unreactive by $S_{N}2$, pushing them toward the $S_{N}1$ route instead.